The present invention relates to a polyvinyl alcohol-based reversibly hydrated gel and highly water-absorptive resin as well as a method for the preparation thereof. The reversibly hydrated gel or highly water-absorptive resin is useful as materials for medical treatment and sanitation, materials used in civil engineering and building works, cooling media or cold-keeping agents and so on.
It is known that polyvinyl alcohol, hereinafter referred to as PVA, forms a hydrated gel when an aqueous solution of PVA is subjected to repeated cycles of freezing and thawing or partially dehydrated under reduced pressure in a frozen state (see, for example, Japanese Patent Kokai 57-130542 and 59-56446 and U.S. Pat. No. 4,472,542). Although a hydrated gel of PVA is a rubbery material having excellent elasticity, tensile strength, water-resistance, oil-resistance and other desirable properties, it has several disadvantages, for example, that the water contained in the gel is gradually lost when the gel is prolongedly kept standing to cause disappearance of the desirable properties inherent in the hydrated PVA gel, that the hydration of the PVA gel is irreversible so that a PVA gel once dehydrated can no longer absorb water to return to the hydrated state and that the gelled structure of a hydrated PVA gel is dissolved when the gel is heated at about 80.degree. C. or higher to give a mere aqueous solution of PVA which cannot return to a hydrated gel upon cooling.
Accordingly, it is eagerly desired to develop an improved PVA-based hydrated gel without the above described disadvantages in the conventional hydrated PVA gels.